Amyotrophic lateral sclerosis (ALS), familiarly known as Lou Gehrig's disease, is the most common adult motor neuron disease, affecting approximately 30,000 persons in the United States (Bruijn et al., 2004). About 90% of ALS cases are sporadic and the other 10% occur as a result of an inherited mutation (Boillee et al., 2006). Regardless of the manner in which ALS is acquired, similar symptoms characterize the progression of the disease. The symptoms include the denervation of target skeletal muscles through the selective loss and degeneration of motor neurons, which leads to muscle atrophy and paralysis in the limbs and respiratory muscles. Although ALS is the most common motor neuron disease, there is no cure or effective treatment that can prevent the loss of motor neurons or significantly improve survival after diagnosis. The identification of signaling pathways and downstream molecules that regulate the initiation and progression of ALS remains a significant challenge in the search for novel therapeutics (Dunckley et al., 2007).
The transcriptional and post-translational regulatory networks that control neuromuscular synapse assembly and maintenance have been well characterized (Sanes and Lichtman, 2001); however, a role for post-transcriptional mechanisms in regulating this process has not been described. In this regard, microRNAs (miRNAs or miRs) are being recognized as major post-transcriptional regulators of many biological processes (Bartel, 2004; Van Rooij et al., 2007a). MiRNAs are small, non-protein coding RNAs of about 18 to about 25 nucleotides in length that regulate gene expression in a sequence-specific manner. MiRNAs act as repressors of target mRNAs by promoting their degradation, when their sequences are perfectly complementary, or by inhibiting translation, when their sequences contain mismatches.
MiRNAs are transcribed by RNA polymerase II (pol II) or RNA polymerase III (pol III; see Qi et al. (2006) Cellular & Molecular Immunology Vol. 3:411-419) and arise from initial transcripts, termed primary miRNA transcripts (pri-miRNAs), that are generally several thousand bases long and are derived from individual miRNA genes, from introns of protein coding genes, or from poly-cistronic transcripts that often encode multiple, closely related miRNAs. See review of Carrington et al. (2003). Pri-miRNAs are processed in the nucleus by the RNase Drosha into about 70- to about 100-nucleotide hairpin-shaped precursors (pre-miRNAs). Following transport to the cytoplasm, the hairpin pre-miRNA is further processed by Dicer to produce a double-stranded miRNA (Lee et al., 1993). The mature miRNA strand is then incorporated into the RNA-induced silencing complex (RISC), where it associates with its target mRNAs by base-pair complementarity. In the relatively rare cases in which a miRNA base pairs perfectly with an mRNA target, it promotes mRNA degradation. More commonly, miRNAs form imperfect heteroduplexes with target mRNAs, affecting either mRNA stability or inhibiting mRNA translation. Loss of function mutations in vertebrates have demonstrated that miRNAs are key regulators of diverse biological processes including cardiac hypertrophy, heart morphogenesis, and lymphocyte development (Van Rooij et al., 2007b; Zhao et al., 2007; Xiao et al., 2007). However, the relationship of miRNAs to neuromuscular synapse function and signaling remains to be established.